Method of reforming reformable members of an electronic package and the resultant electronic package

ABSTRACT

An electronic package includes a substrate having a contact pad thereon, a reformable member such as a solder ball positioned on the contact pad, and an elastic member positioned around the reformable member. The elastic member exerts a girdling force on the reformable member so that when the reformable member is softened, the elastic member elongates the reformable member. This elongation accommodates thermal and other stresses between the foregoing substrate and another substrate joined at the free end of the reformable member. An apparatus is also provided for positioning the elastic member on and around the reformable member.

FIELD OF THE INVENTION

The invention relates generally to electronic packages and moreparticularly to assemblies for connection of integrated circuit chips tochip carriers, chip carriers to printed circuit boards, modules toprinted circuit cards, and the like.

BACKGROUND OF THE INVENTION

Various ceramic ball grid array packages, leadless chip carriers,laminated chip carriers, tape ball grid array packages, and overmolded,globtop, and plastic ball grid array packages are known today. Suchpackages typically contain a semiconductor device (a chip) and providefor electrical and thermal connection paths to and from the chip, whileprotecting the chip from extraneous environmental factors, processingchemicals, and handling.

Such packages typically include connections known in the art as solderreflow (or melt) connections. These connections provide for bothelectrical and thermal conduction paths. It is known from prior art thata reformable solder ball 107 can be attached to contact pad 105 onsubstrate 103 of electronic package 101, as depicted in FIG. 1. Further,as depicted in FIG. 2, it is known from prior art that solder ball 207can be attached to contact pad 206 of substrate 204. Typically substrate204 is circuitized. Solder ball 207 provides a standoff distance sbetween contact pads 205 and 206; this standoff provides mechanicalcompliance between the two substrates. It is also known that a higherstandoff, with resulting increased compliance, can result in improvedresistance to fatigue. The connections may be formed between a chip anda circuitized substrate, serving as a chip carrier package. Examples ofcircuitized substrates include printed wiring boards, flexible circuitcards, laminated chip carriers, metallized ceramic substrates, andmulti-layer ceramic substrates. When a chip is connected to acircuitized substrate with numerous solder reflow connections, this formof connection is also known in the art as “flip-chip” or controlledcollapse chip connection (C4), and results in a flip chip package.

Aside from flip chip packages, there are many kinds of chip carrierpackages known in the art of semiconductor packaging. Other forms ofchip carrier packages include wirebond chip carriers, thermo-compressionbond chip carriers, and hybrid chip carrier devices. Further connectionof a chip carrier package to a circuit card or board may be accomplishedby utilizing known solder reflow methods to obtain what is commonlyknown as a ball grid array (BGA) connection therebetween. Variouscircuit cards, usually comprised of several layers of dielectricmaterial, e.g., fiberglass-reinforced epoxy resin, interspersed withvarious conductor layers, (power, signal and/or ground planes) and oftenincluding plated through-holes and/or internal conductive vias, areknown in the art.

An inherent problem with either the C4 or BGA connection is that thethermal expansion characteristics of the two objects being connected maydiffer substantially. For example, the linearized coefficient of thermalexpansion (CTE) of silicon is in the 2.5 to 4.0 part per million/degreeCelsius (ppm/C.) range, while the CTE of a circuitized substrate may be3 to 25 ppm/C., depending on the material choices. Therefore, the solderconnections between a chip and circuitized substrate will be subject tothermally-induced stresses. This stress is generally increased by theuse of larger, more complex, higher performance (i.e., more powerful andthermally dissipative) and higher signal count chips. Since manyhundreds, and even thousands, of connections may be present in a singlechip carrier package, the method of forming the connections must also bereliable, manufacturable, efficient, and economical. In a similarmanner, thermal expansion mismatch between a chip carrier package and acircuit card or board may give rise to significant stresses whichadversely affect reliability. (A typical printed circuit board composedof common glass-epoxy has a CTE value of 17 to 22 ppm/C., while a chipcarrier has a composite CTE value which may vary from 3 ppm/C. to 25ppm/C., depending on geometry and material choices.)

To manage these problems, several strategies are known in the art. Twoof these include the use of a stress-relieving underfill (orencapsulant), and the use of high-standoff solder joints includinghigh-melt solder standoffs and solder-coated copper ball standoffs. Theuse of fatigue resistant solder materials, the use of conductiveadhesives to form the C4 or BGA connections, and the use of compliantsubstrate materials are also known in the art. Each of these methodshelps to form reliable connections, but at a cost of manufacturability,restrictive material choice, or other disadvantage. For example, the useof underfill as a stress-relieving method to protect the connectionsgenerally prevents subsequent removal and replacement to replace orrepair a device (commonly known as rework). The use of a solder-coatedcopper ball as a standoff-enhancement device results in a higherstandoff, but greatly increased joint stiffness. Known solder columnscomprise preformed high-temperature solder material. Thehigh-temperature material is required to keep its preformed shape duringsubsequent solder reflow attachment. The solder column is generallyattached to a device with a second, lower-melting solder in conjunctionwith an alignment fixture. The manufacturability difficulties of thesolder column connection method limit the pitch (distance betweenadjacent columns) of a package interconnection to about 1.0 millimeter(mm) at the present time, and require the use of a hierarchy of soldermelt materials.

A method for dynamically forming a solder column instead of preformingthe solder column, while retaining a column-like shape duringtemperature excursions beyond the solder melt temperature would be verydesirable. Further, the ability to obtain shapes other than simplecolumns would be very desirable because the shape of the solderconnection can influence the internal stresses, and the reliability ofthe connection and manufacturability.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide an electronic packagehaving an improved standoff capability to provide improved packagereliability.

Another object of the present invention is to provide a fixture forpreparing the foregoing electronic package for assembly manufacturing.

Still another object of the present invention is to provide a method forpreparing the foregoing electronic package for manufacturing.

According to one aspect of the present invention, an electronic packageis provided comprising a substrate having a contact pad thereon, areformable member positioned on the contact pad, and an elastic memberpositioned substantially around the reformable member.

According to another aspect of the present invention, there is providedan apparatus for positioning the elastic member, substantially around areformable member with the reformable member positioned on a base. Theapparatus comprises a first plate having a pin projecting into anopening of the elastic member and a second plate having an aperturetherein and interposed between the first plate and the elastic member.The pin projects through the aperture in the second plate. The secondplate is movably positioned with respect to the pin of the first platefor removing the elastic member from the pin and positioning the elasticmember substantially around the reformable member such that thereformable member is located in the opening of the elastic member.

The invention is adaptable to mass production and will providesignificant improvement in the ability to manufacture high reliabilityelectronic packages, having a high standoff from the substrate to whichthe packages are assembled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with prior art.

FIG. 2 is a partial side sectional view, in elevation and on a muchenlarged scale, of the electronic package of FIG. 1 connected to acircuitized substrate in accordance with the prior art.

FIG. 3 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package with a reformable solder ballpositioned to receive an elastic sleeve in accordance with the presentinvention.

FIG. 4 is a partial side sectional view, in elevation and on a muchenlarged scale, of the electronic package of FIG. 3 after the elasticsleeve has been positioned around the reformable solder ball.

FIG. 5 is a partial side sectional view, in elevation and on a muchenlarged scale, of the electronic package of FIG. 4, depicting theelastic member positioned around a reformed solder slug.

FIG. 6 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with anotherembodiment of the present invention, depicting elastic sleevessubstantially surrounding reformable solder balls positioned near acircuitized substrate.

FIG. 7 is a partial side sectional view, in elevation and-on a muchenlarge scale, of the electronic package of FIG. 6 after heating and incontact with the circuitized substrate after heating and contact.

FIG. 8 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with yet anotherembodiment of the present invention, depicting elastic sleevessubstantially surrounding reformable solder balls.

FIG. 9 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with stillanother embodiment of the present invention depicting reformed solderslugs in a substantially hourglass shape.

FIG. 10 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with anotherembodiment of the present invention, depicting elastic sleevessubstantially surrounding reformable solder columns.

FIG. 11 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with anotherembodiment of the present invention, depicting elastic sleevessubstantially surrounding reformable solder balls.

FIG. 12 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with anotherembodiment of the present invention, depicting elastic sleevessubstantially surrounding reformed c shaped solder slugs.

FIG. 13 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with anotherembodiment of the present invention, depicting an assembly of elasticsleeves for positioning over reformable solder balls.

FIG. 14 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with anotherembodiment of the present invention, depicting the assembly of elasticsleeves shown in FIG. 13 positioned over and around the reformablesolder balls.

FIG. 15 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with anotherembodiment of the present invention, depicting the assembly of elasticsleeves of FIG. 13 positioned around reformable solder balls withreformable solder balls being shaped by elastic sleeves to form reformedsolder slugs.

FIG. 16 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package in accordance with stillanother embodiment of the present invention, depicting the reformedsolder slugs shown in FIG. 15 after removal of the elastic sleeves. Alsoshown is an apparatus for further positioning other elastic sleeves onthe reformed solder slugs.

FIG. 17 is a partial side view, in elevation and on a much enlargedscale, depicting an alignment fixture in accordance with the presentinvention for positioning elastic sleeves on reformable solder balls.

FIG. 18 is a partial side sectional view, in elevation and on a muchenlarged scale, depicting elastic sleeves with a connector positioned onreformable solder balls in accordance with the present invention.

FIG. 19 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package with several reformable solderslugs in accordance with still another embodiment of the presentinvention and an assembly of coiled elastic sleeves for positioning overthe reformable solder slugs.

FIG. 20 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package with several reformable solderballs in accordance with another embodiment of the present invention,depicting an assembly of coiled elastic sleeves and tapered pins forprestressing and positioning the elastic sleeves over the reformablesolder balls.

FIG. 21 is a partial side sectional view, in elevation and on a muchenlarged scale, of an electronic package with several reformed solderslugs in accordance with another embodiment of the invention and coiledelastic sleeves.

FIG. 22 is a partial side sectional view in elevation on a much enlargedscale depicting elastic sleeves with a connector positioned onreformable solder balls positioned on contact pads by the weight of atool.

BEST MODE OF THE INVENTION

Referring now to the drawings in detail, FIGS. 3-5 illustrate a processfor making an electronic package according to one embodiment of thepresent invention. The electronic package 301 comprises a substrate 303,a contact pad 305, and a reformable ball 307. By way of example,reformable ball 307 is a solder ball. An elastic sleeve 309 is formedfrom rubber, plastic or a shaped memory alloy and is initially apartfrom electronic package 301. When relaxed, an inner diameter (shown inphantom) of elastic sleeve 309 is less than an outer diameter ofreformable solder ball 307. In FIG. 4, elastic sleeve 309 has beenpositioned around reformable solder ball 307 by stretching the elasticsleeve from its relaxed state to an unrelaxed state conforming to theoutside diameter of the reformable solder ball 307. To ease applicationof the elastic sleeve, a lubricant such as a solder flux may beemployed. A girdling (or squeezing) pressure now exists between elasticsleeve 309 and reformable solder ball 307, while ball 307 is in a solidcondition. By heating reformable solder ball 307 to its softeningtemperature, the solder ball responds to the girdling pressure exertedby elastic sleeve 309. Consequently, reformable solder ball 307 takes anew elongated shape depicted in FIG. 5 as reformed, nipple-shaped solder507, which shape is predetermined and held by now-relieved elasticsleeve 309. As seen in FIG. 4, elastic member 409 contacts the sideportion of reformable member 407 and not the bottom portion thereof. Thebottom portion is, of course, the portion which is directly positionedon and electrically coupled to an external conductor (not shown). Afterreshaping, the stiffness of the reformed solder 507 may be increased byreducing temperature. In particular, an increased standoff distance h′between contact pad 305 and the tip of reformed solder 507 is evident(as compared to distance h in FIG. 3).

The method for making and utilizing an elongated reformed slug may bevaried somewhat depending on the materials chosen, so several exampleswill be given to illustrate how to best implement the present invention.

Softening of many materials such as plastics, epoxies, metals, andorganic materials is known to occur at increased temperature. With somematerials, such as solder, a phase change from solid to a melt or moltenstate of the material can be achieved at a sufficient temperature. Themolten state exhibits essentially a zero shear modulus. Eutectictin-lead solder, for example, has a melting phase transition point ofabout 183° C. (Centigrade). Higher melting point solders can be obtainedwith various ratios of tin and lead; about 3% tin and about 97% lead isa common high-melt (about 310° C.) alloy.

Numerous plastics exhibit a phase transition from hard, glass-likebehavior to relatively soft, visco-elastic behavior at a temperature,known as the glass transition temperature, in the range of about −70° C.to about 170° C. and have a corresponding, approximately ten timesdecrease in stiffness through that transition. These materials whenfilled with conductive particles can be used as reformable balls orslugs in conjunction with appropriate choices of materials for elasticmembers. For example, epoxy resins, which can have electricallyconductive filler particles such as silver mixed in with the resin canhave glass transition temperatures below about 100° C.

The method of making electronic package 701 with a high standoffreformed slug 707A illustrated in FIG. 7 will now be described. FIG. 6illustrates an elastic sleeve 609A which has been stretched onto andaround reformable ball 607A prior to any heat being applied. The finalshape and diameter of the opening 737A (FIG. 7) of the elastic sleeve609A on reformed slug 707A are carefully chosen and predetermined inconjunction with both the quantity of material in the reformable ball607A and the contact pad 605A geometry, so as to obtain a desirablefinal shape of reformed slug 707A. For example, for a circular contactpad with diameter of about 24 mils (thousandths of an inch), and aeutectic solder volume of 10000 cubic mils of solder, the cylindricalsleeve would have an interior diameter of about 22 to 23 mils, with aheight of about 10-15 mils, and a wall thickness of about 2 to 4 mils.For this configuration, the maximum initial diameter of the reformableball 607A is about 28 mils, so that the elastic sleeve 609A iscircumferentially stretched to be positioned around reformable ball607A, applying pressure onto the reformable ball. When the reformableball is comprised of eutectic solder, before the heat is applied, thepressure of the elastic sleeve 609A has negligible effect on the shapeof reformable solder ball 607A. An excellent and common material choicefor reformed slug 707A is eutectic solder because it is bothelectrically and thermally conductive, has a relatively low meltingpoint of about 183° C., has a high surface tension and easily wets toand connects to metallic contact pads 705A and 706A. In order to providethe high standoff of reformed slug 707A with a minimum quantity ofsolder and small contact pad dimensions, elastic sleeve 609A should beshaped as a hollow cylinder and comprise low durometer (less than about70 Shore A) silicone rubber. Silicone rubber is resistant to hightemperature (up to about 300° C. or more for short period of time, e.g.,less than several minutes) without degradation. It also has desirablestiffness properties, because it is relatively soft (a modulus of a fewhundred psi) compared to eutectic solder in solid phase (a modulus ofabout a million psi, depending on temperature) but is stiff compared tomolten solder (with nearly a zero modulus, as it is a fluid). Othermaterials for the reformable slugs and elastic sleeves may be used aswell. For example, soft plastic which shrinks upon heating may readilybe obtained in the form of a cylindrical sleeve, and utilized to applygirdling pressure to a reformable slug in the form of a silver-filledconductive epoxy adhesive.

Reformable solder ball 607A with elastic sleeve 609A is positioned nearanother substrate 604 having contact pad 606A, and the temperature ofthe reformable solder ball 607A is increased to melt the reformablesolder ball. The melting of the solder allows wetting of reformablesolder ball 607A to the second contact pad 706A. The melting reduces thereformable solder ball 607A material stiffness sufficiently so that thepressure exerted by elastic sleeve 609A is sufficient to now force achanged shape, a cylinder depicted in FIG. 7 with opening 737A. Thegirdling pressure exerted by elastic sleeve 609A easily overcomessurface tension, and forces what would have been a truncated ball-likeshape (as depicted in FIG. 2 of reformable ball 107) into a more uprightcylindrical shape 707A. For the same quantity of solder and paddimensions, a higher standoff height (s′>s) will result. Thepredetermined shape of the opening 737A of elastic sleeve 709A, as thatof a cylinder, will nearly be replicated onto the molten reformed solderslug, as the resistance of the surface tension with respect to thematerial stiffness of the silicon rubber of elastic sleeve 709A is verysmall. Portions 710A of the surface of reformable slug 707A which arenot in contact with the elastic sleeve 709A will take a rounded shape asdictated by surface tension. The transformation of shape occurs veryquickly (on the order of seconds upon melting of the solder) and uponcooling, the transformed shapes are maintained. The elastic sleeve 709Amay be left in place, because the stiffness of the rubber comprising theelastic sleeve is small compared with solidified eutectic solder and nosignificant stress concentrations or detriments due to its presence willresult. However, there is a very significant fatigue life improvementrealized for reformed slug 707A due to the increased standoff, s′, ofthe connection 702. Even though the example presented here has describedone attachment between substrates 603 and 604, it can be easily shownthat multiple attachments can be formed as shown in FIGS. 6 and 7. Forexample, elastic sleeve 609B acts to yield another elongated reformedslug 707B which is positioned on contact pad 705B of electroniccomponent 701 and contact pad 706B on substrate 704. When there aremultiple attachments on the electronic package, the present inventionalso reduces the tendency of reformable slug 707A to accidentallycontact and ‘bridge’ to another reformable slug 707B to form anundesirable short circuit.

FIG. 9 illustrates another reformed solder slug 907 formed substantiallyin the shape of an hourglass in accordance with another embodiment ofthe present invention. FIG. 8 illustrates the manner of formation. Anelastic donut-shaped sleeve 809 is stretched around reformable ball 807which is attached to a contact pad 805 of a substrate 803. Reformableball 807 is initially in a solid form. Sleeve 809 provides a girdlingpressure of approximately several pounds per square inch on the solderball when the solder ball is in this solid, spherical form. The girdlingpressure is not large, but is sufficient to reform the softened solderball. Then, solder ball 807 is heated to its softening temperature andthe elastic sleeve relaxes, forcing solder material outward from theinterior of the sleeve. Portions 910 of reformed solder slug 907 whichare not in contact with elastic sleeve 909 take a smoothed bulgingshape.

The present invention allows electronic package designers to have manyshape options available to optimize reliability of various electronicpackages. Utilizing other shapes of openings of elastic sleeves tofurther tailor the resulting shape of the reformable ball or slug makesthis possible. The elasticity of silicon rubber, approaching severalhundred percent strain allows an elastic sleeve to be molded into avariety of complex and intricate shapes, even with substantiallyundercut features. In FIG. 10, an example is depicted of an opening 1037of elastic sleeve 1009 which has been used to form reformed slug 1007into a shape resembling a helix. FIG. 10 illustrates three differentcross sections of the sleeve 1009. Other shapes are possible, forexample, a C-shaped reformable slug 1209 is depicted in FIG. 12.S-shaped, and even coil-shaped reformable slugs and many other usefulshapes can be obtained with complementary elastic sleeve shapes.

A complex ball grid array electronic package may require several hundredor even several thousand successful electrical connections, but in somedevices, e.g., ceramic substrate electronic packages, not all of theconnection members are highly stressed during operation. Typicallyseveral dozen of the connections near the far corners are highlystressed, while the interior connections are not. The stress tolerantslugs of the present invention can be used on the highly stressed cornerconnections to relieve radially oriented shear stresses which resultfrom thermal expansion mismatches in the electronic package. Shear andbending stresses within the connection member are driven by the radiallyoriented thermal expansion mismatches. Since shear and bending stresseswithin the connection member increase with member section thickness, itis clearly beneficial to reduce member section thickness. Member sectionthickness is defined as the diameter of a round connection member.However, it is only necessary to reduce member section thickness in onedirection; that is, with respect to the radially oriented direction. Arectangularly or eliptically-sectioned hourglass shaped connection, withthe thinnest bending axis (minor axis) oriented radially from theelectrical package central point, substantially reduces shear stress forthose connections. This configuration allows the connection member tohave reduced stresses while retaining the greatest possible crosssectional area, so as to have the greatest possible tensile strength.The interior solder slugs do not need elastic sleeves surrounding them,because these solder slugs, when reformed will have increased standofffrom standoff caused by the several dozen corner connections which havethe elastic sleeves. In other words, not all of the package connectionsneed to have an elastic sleeve on them to achieve the benefits of thepresent invention. Application of the elastic sleeves to a sufficientnumber of reformable slugs to overcome surface tension of all the slugswill create increased standoff for all. It is also possible to use theelastic sleeves to simply enhance standoff and not for electrical orthermal connection, by strategically placing reformable balls or slugsto uniformly “lift” the substrate.

Composite molding many elastic sleeves together with an internalalignment web as one part is most convenient and efficient to quicklyand simply position thousands of elastic sleeves in one operation. InFIG. 11, an elastic sleeve 1109A is positioned around reformable ball1107A on contact pad 1105A on substrate 1103. Another elastic sleeve1109B is shown similarly positioned around another reformable ball 1107Bwhich is on another contact pad 1105B on substrate 1103 of electronicpackage 1101. The elastic sleeves 1109A,B are connected together by webportion 1129, which aligns the sleeves with the reformable balls onsubstrate 1103. Positioning tool 59 (shown in two positions) withprojections 61 has been used to press and position elastic members1109A,B and other elastic sleeves 1109C, etc.onto and around reformableballs 1107A,B etc. Since the elastic sleeve openings are molded inpredetermined locations so as to align to and correspond to selectedones of reformable balls, it is a simple and expedient matter to pressthe composite mold into place.

FIGS. 13 through 15 illustrate another embodiment of the presentinvention, an assembly 1302 of elastic sleeves 1309 and base plate 1320for positioning the sleeves over reformable balls 1307. A portion ofassembly 1302 can be positioned onto and around reformable balls 1307.Openings 1337 of the sleeves are aligned with reformable balls 1307 oncontact pads 1305 on substrate 1303 of electronic package 1301. It ispossible to have variously shaped and oriented openings 1337 in thesleeves, so that the shapes of reformable slugs on the electronicpackage are optimized for the best reliability of the electronic packageattachment. One material that can be used for the sleeves is siliconrubber. The base plate 1320 secures the sleeves in place and can becomprised of a metal, preferably copper. Base plate 1320 includespressure relief openings 1322 depicted as holes through the base plate1320 to each opening 1337.

In FIG. 14, the openings 1337 (shown in phantom) of the sleeves ofassembly 1309 are now positioned around respective reformable balls1307, with opening 1337 now stretched to larger opening 1437, prior tosoftening of reformable balls 1307. In FIG. 15, reformed slug 1507 isformed by softening of reformable ball 1307 so as to yield to girdlingpressure exerted by elastic sleeves 1337. Opening shape 1537 is forcedupon reformed slug 1507. Portions 1510 of the elastic sleeves, not incontact with elastic sleeve, take a rounded shape as dictated by thesurface tension of the softened material of reformed slug 1507. Thisconfiguration is held until reformed slug 1507 is re-hardened,preferably by cooling. In FIG. 16, the resulting electronic package 1601is depicted with the reformed slug 1507 after assembly 1309 is removed.

Apparatus 1639 can provide for alignment and easy positioning of elasticsleeves 1609. This apparatus includes a rigid first plate 1643 whichaligns and holds a number of pins 1645. These pins are positioned tocorresponded to positions of respective reformed slugs 1507 which arepositioned on contact pads 1305 on substrate 1303 of an electronicpackage 1601. The elastic sleeves 1609 are loaded onto the pins, whichthen provide alignment and guidance, and are pressed onto and aroundcorresponding reformed slugs. A connector web or sheet 1629, depicted inphantom, is sheared off at tearaway tabs 1621, leaving separated elasticsleeves 1609 on the pins. Plate 1639 is used to push and dispense theelastic sleeves 1609. A second plate 1639 has apertures 1649 throughwhich the pins 1645 extend, and is moveable so that second plate 1639can force the elastic sleeves 1609 out and around the reformed slugs.The second plate 1639 is removed after the elastic sleeves are inposition. Each of pins 1645 can be coated with a region 1653 oflow-friction material, preferably Teflon (Teflon is a registeredtrademark of E.I. Dupont de Nemours and Co.) but alternately solderflux, plastic, or other low friction material can be used. Numerouselastic sleeves can be molded together as a one-piece part, withconnector 1629 providing alignment between one another. When areformable slug is shaped as a tapered cone, as reformed slug 1507,there will be little stretching, and little stress in the elasticsleeve. Upon temperature induced melting, the solder slug will assumethe same final shape of the reformed slug 907 shown in FIG. 9. It isimportant that the reformable slug wet to the contact pad 906 before theelastic sleeve releases any stored stress, otherwise undesirablegeometries may occur. A quantity of lower-melting solder paste or otheradhesive may be used to provide sufficient wetting contact at thecontact pad 906.

For some applications, reformable balls which are not initially bondedto contact pads can be shaped with elastic sleeves according to thepresent invention. For example, if the reformable balls are made of amaterial having little surface tension or are difficult or impossible toadhere to an electronic package, reformable balls can be formed intodesired shapes independent from the electronic package. In this case, aremovable anisotropically conductive interposer may be needed. This isuseful for various chip and wafer testing procedures. One example ofthis type of processing is illustrated in FIGS. 17 and 18. In FIG. 17,reformable balls are positioned on a base 1741, with an alignmentfixture 1735 in conjunction with a connector 1729 which interconnectselastic sleeves 1709A, B and C. The connector 1729 remains connected tothe elastic sleeves as the elastic sleeves 1709A,B, and C are pressedonto and around the reformable balls 1707A, B and C respectively. A base1741, with recesses 1751, provides for alignment of the reformable ballsand is used in conjunction with a series of pins 1745A, B and C whichare held and located by a first plate 1743. As shown in FIG. 18, elasticsleeves 1709A, B and C have been positioned around reformable balls1707A, B, and C, using the alignment fixture 1735, with the elasticsleeves being individually circumferentially stretched around thereformable balls. The girdling pressure provides friction which holdsthe reformable balls in place. The openings 1837A,B and C of the elasticsleeves 1709A,B and C conform to a rounded, ball-like reformable ballshape to provide retention force. In this prestressed, aligned state, alarge sheet of an array of reformable balls may be easily handled,stored, transported, positioned, protected, inspected, and tested.Reformable balls may be removed, replaced as necessary, and prepared foruse in joining an electronic package to another substrate. In manyelectronic packages, the ‘pitch’, or distance between one reformableball to another is a constant (typically about 1.27 mm or about 1.00 mmfor ball grid array packages) and the pattern of the placement of theballs is a simple square array. For these packages, which account for alarge percentage of applications, it is possible to prepare large sheetsof elastic sleeves with reformable balls pre-loaded. The desiredquantity and size of reformable balls may simply be cut from the sheetas needed for various sized packaging applications. In this manner, theelastic sleeves connected by connector 1729 also become a usefulpackaging method for handling and transporting large numbers ofreformable balls. A roll-like continuous process may be implemented tomanufacture, store large quantities economically. The material chosenfor the elastic sleeve, preferably cured silicon rubber, can maintainthe circumferential prestress for a long period of time.

Connecting the sheet of reformable balls to an electronic package may beaccomplished in one of two manners, depending on whether or not theconnector 1729 is to remain in place in the final configuration. If itis desirable to remove the connector, the sheet of reformable balls withelastic sleeves attached may be positioned on contact pads 2205A, B andC of an electronic component 2203 and heated so as to melt thereformable balls releasing the elastic sleeve prestress. To avoid havingsurface tension push the elastic sleeves away from the component, tool59 may be used in conjunction with gravity weight as shown in FIG. 22 tobe certain that elastic sleeve will remain in place and provide enhancedstandoff as described earlier. Subsequently, the connector 2229 may beremoved by shearing it off at tearaway features 2221. The electronicpackage is then reflowed to another substrate with the elastic sleevesleft in place to provide enhanced standoff and shape control.

In some applications there is no need to remove the connector betweenelastic sleeves. To avoid surface tension pushing the elastic sleevesout of place, it is necessary to make wetting contact between each ofthe contact pads on the electronic package and the reformable ballsbefore elastic stress stored in the elastic sleeves is released, i.e.,before the reformable balls are molten. This is simply accomplished bythe use of a quantity of lower temperature melting material which isadhered or wetted to each reformable ball and each contact pad using,for example, a lower-melting solder paste in conjunction withapplication methods well-known in the industry (i.e. stencilapplications). The solder paste may be rapidly applied to the one orboth sides of each reformable ball while held in the sheet format.

If the reformable ball is chosen to be of a higher-melting temperaturesolder than the solder paste, the elastic sleeve, left in place, canserve as a protective barrier in conjunction with the shaping purpose ofthe present invention. For example, if a high-tin content solder paste(e.g., a eutectic paste) is used on the side of the reformable ballopposite the contact pad of an electronic component for attachmentpurposes, it may be desirable to maintain separation of the tin from theelectronic component and the solder paste on the contact pad. Tin ismetallurgically active with various other metals, and can rapidly attackand alter contact pad metallurgy and so form undesirable reactionbyproducts. The elastic sleeve of the invention will prevent tin fromwetting up the reformable ball during reflow and consequently willprevent the reactive tin attack on the contact pad of the electroniccomponent. In a similar manner, silver, corrosive flux, or other harmfulchemicals may be prevented from migration across the protective barrierformed by the elastic sleeve.

The present invention may also be utilized to form reformable slugs inthe shape of a cone or cylinder. If the reformable slugs are made ofhigh-melt solder, it is possible to simply leave the cone or cylindricalshapes in place on the electrical component, making use of eutecticsolder paste or lower melting point solder to effect an electricalconnection. A distinct advantage of the invention over prior art methodsof casting or molding is that undercut features, such as hourglassshapes, may be readily produced on reformable slugs. Hourglass shapesare known to have more optimal shear stress distribution characteristicsthan cylinders, cones, or balls.

A coil of soft aluminum wire, coated with polyimide or other material towhich the reformable slug will not wet, may also be used as an elasticsleeve as depicted in FIG. 19. The coil 1909 is slipped into place overa tapered reformable slug 1907 and connector 1929 is removed. A coil ofa shape memory alloy wire can also be utilized as elastic sleeve 2009,as illustrated in FIG. 20. A shape memory alloy is a material exhibitingthe characteristic of being able to return to a particular shape,despite significant deformations, upon heating to a temperature above acritical range known as an memory activation temperature range. Knownmaterials which exhibit this characteristic include nickel-titaniumalloys. There is some hysteresis involved in the actual deformation andshape return paths followed, over a range of temperatures spanning about20° C. to about 60° C. but essentially the original shape is recoveredby heating. The memory activation temperature range which activates thematerial microstructural changes and cause the recovery (which typicallyspans just a few degrees C.) of the material to its original shape maybe tailored to be in a desired range by choice of alloy constituents ofthe material. A memory activation temperature range of about −100° C. toabout +100° C. is available with known nickel-titanium materials, anddeformations from the original shape involving material strains up toabout 6 or 8 percent are possible to recover from. The wire may becoated with a material to prevent wetting of molten material of thereformable ball or slug. For example, a polyimide coating would preventeutectic solder from wetting to it. A wire of this material may beformed into a coil by wrapping about a mandrel and held there whileannealed (typically at about 400° C. to about 500° C. for a few minutes)so as to become ‘memorized’ as the ‘original’ or ground state for thematerial. The dimensions of this ground state are carefully chosen inconjunction with the quantity of material used for the reformable ballor slug and the desired final dimensions of the transformed shape of thereformed slug. Then, the coil, which comprises the elastic sleeve 2009,may be stretched and deformed so as to have a greater diameter than thereformable ball or slug. As shown in FIG. 20, tapered pins 2045 can beused to accomplish this. The prestretched elastic sleeve is thenpositioned around the reformable ball or slug. The wire and reformableball or slug are then heated to a temperature which activates the shapememory alloy to constrict around the reformable ball or slug and squeezeit into a useful shape. Use of a coil wound one or more times around areformable slug or ball can result in high aspect ratio shapes 2107, asdepicted in FIG. 21. A coil with a memorized shape which is not of auniform diameter may also be used, to create a reformable slug shapewhich is more useful (e.g., an hourglass shape). After memoryactivation, the shape memory alloy acts as a ‘normal’ material with astiffness of about 5 Mpsi (million pounds per square inch) and CTE ofabout 6 to 11 ppm/C. A wire diameter of about 1 to about 3 mils coiledinto a nominal diameter of about 10 to about 28 mils may typically beutilized in conjunction with contact pads of about 10 to about 30 milsdiameter. In a manner similar to as described earilier, more than oneelastic sleeve may be connected with a connector such as a sheet ofwater soluable paper or removable film of plastic.

Based on the foregoing, apparatus and methods for forming high stand-offor stress accommodating interconnect slugs have been disclosed. However,numerous modifications and variations may be provided without deviatingfrom the scope of the present invention. For example various materialsfor reformable member, elastic member, or other portions of theinvention may be substituted; shape designs may be changed so as tooptimize results for specific electronic package needs; and methods maybe combined and/or sequentially employed so as to form usefully shapedreformable connection members. Therefore the invention has beendisclosed by way of illustration and not limitation, and referenceshould be made to the following claims to determine the scope of thepresent invention.

We claim:
 1. An electronic package comprising: a substrate having acontact pad thereon; a reformable member having at least one sideportion and a bottom portion positioned on said contact pad, said bottomportion adapte or being positioned on and electrically coupled to anexternal conductor; and an elastic member positioned substantiallyaround only said at least one side portion and in contact with saidreformable member, said elastic member providing a girdling pressure onsaid reformable member.
 2. The electronic package of claim 1 whereinsaid substrate comprises silicon or silicon-germanium.
 3. The electronicpackage of claim 2 wherein said substrate is an integrated circuit. 4.The electronic package of claim 1 wherein said substrate comprisesceramic, epoxy, epoxy-glass, Teflon, polyimide, alumina, glass-ceramic,Kevlar, liquid crystal polymer, glass, fiberglass, or plastic.
 5. Theelectronic package of claim 1 wherein said substrate is a chip carrier.6. The electronic package of claim 1 wherein said substrate is a circuitcard.
 7. The electronic package of claim 1 wherein said reformablemember is bonded to said contact pad.
 8. The electronic package of claim1 wherein said reformable member is electrically conductive.
 9. Theelectronic package of claim 1 wherein said reformable member comprisessolder.
 10. The electronic package of claim 1 wherein said reformablemember comprises a material selected from the group consisting ofelectrically conductive epoxy, silver, gold, tin, lead, bismuth,aluminum and copper.
 11. The electronic package of claim 1 wherein saidreformable member comprises a thermally dependent material.
 12. Theelectronic package of claim 11 wherein said thermally dependent materialhas transition temperature in a range of about 40° C. to about 340°Centigrade.
 13. The electronic package of claim 1 wherein saidreformable member is substantially in a shape of an hourglass, acylinder, a cone, or a nipple.
 14. The electronic package of claim 1wherein said elastic member comprises an elastic collar that surroundsonly said at least one side portion of said reformable member.
 15. Theelectronic package of claim 1 wherein said elastic member comprisesrubber, plastic, or a shape memory alloy.
 16. The electronic package ofclaim 15 wherein said elastic member generally has a shape of adoughnut, a sleeve, or a coil.
 17. The electronic package of claim 15wherein said rubber comprises silicone rubber.
 18. The electronicpackage of claim 15 wherein said plastic comprises a thermally activatedshrinking plastic.
 19. The electronic package of claim 18 wherein saidthermally activated shrinking plastic has an activation temperature in arange of about 40° to about 340° Centigrade.
 20. The electronic packageof claim 15 wherein said shape memory alloy has a memory activationtemperature in a range of about 40° Centigrade to about 340° Centigrade.21. The electronic package of claim 1 wherein said elastic membercomprises a sleeve which surrounds said reformable member, saidreformable member is elongated.
 22. The electronic package of claim 21wherein said elastic member further comprises a tearable memberremovably positioned on said sleeve.
 23. The electronic package of claim1 wherein said reformable member comprises a solder ball.
 24. Theelectronic package of claim 1 further comprising another contact pad onsaid substrate, another reformable member positioned on said othercontact pad and another elastic member positioned substantially aroundsaid other reformable member.
 25. The electronic package of claim 24wherein said elastic members include a connector therebetween.
 26. Theelectronic package of claim 25 wherein said connector is removable. 27.The electronic package of claim 26 wherein said connector comprises atearable member removably positioned between said elastic members toalign said elastic members with said reformable members.